1. Field of the Invention
The invention relates to a lithographic projection apparatus for imaging a mask pattern in a mask onto a substrate, said apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a mask table provided with a mask holder constructed to hold a mask;
a substrate table provided with a substrate holder constructed to hold a substrate; and
a projection system constructed and arranged to image a portion of the mask onto a target portion of the substrate.
2. Background of the Related Art
A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions which are successively irradiated through the mask, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine. The apparatus may also have more than one mask table and may comprise components which are operated in vacuum, and are correspondingly vacuum-compatible.
Lithographic apparatus may employ various types of projection radiation, such as ultraviolet light (UV), extreme UV (EUV), X-rays, ion beams or electron beams, for example. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be refractive, reflective or catadioptric, for example, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc; for simplicity, such components may be referred to in this text, either singly or collectively, as a xe2x80x9clensxe2x80x9d.
There is a continuing desire in the semiconductor industry to be able to manufacture integrated circuits with ever higher component densities and hence smaller feature size. To be able to image smaller features, the temperature of various parts of the projection apparatus is required to be within a narrow range of a predetermined value. Deviations from the predetermined value would cause expansion or contraction of components, causing errors in the accurate positioning of, for instance, substrate and mask which is required in the projection of the features from the pattern in the mask onto the substrate. Temperature deviations from the said predetermined value may also cause errors in the readings of sensors that are used for, for instance, the positioning and/or leveling of the substrate. Such sensors may be sensitive to changes in the (temperature dependent) refractive index of a volume of gas through which sensor beams of radiation travel. An example of a sensor that is sensitive to refractive index variations is an interferometric displacement measuring means employing sensor beams of monochromatic coherent radiation.
An object of the present invention is to provide a lithographic projection apparatus of which a part that is sensitive to temperature deviations, such as the substrate or mask holder or a volume of gas through which beams of radiation travel, is shielded from another part that may cause the temperature sensitive part to deviate from the predetermined temperature.
According to the present invention there is provided a lithographic projection apparatus for imaging a mask pattern in a mask onto a substrate, said apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a mask table provided with a mask holder constructed to hold a mask;
a substrate table provided with a substrate holder constructed to hold a substrate;
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate;
a heat shield arranged such that a first part of said apparatus is shielded by said heat shield from a second part, said first part required to have a temperature of a predetermined value, and said second part having a characteristic that may influence said temperature of said first part; and
heat shield temperature controlling means constructed and arranged to control a temperature of said heat shield to said predetermined value.
In embodiments of the invention the heat shield is arranged such that it is not in thermal contact with the second part. It may be arranged at a distance from said second part and it may at least partly surround said second part. The heat shield temperature controlling means may comprise a conduit arranged in thermal contact with said heat shield and constructed to circulate a fluid, said heat shield temperature controlling means further being constructed and arranged to control a temperature of said fluid to said predetermined value and to circulate said fluid through said conduit.
The characteristic of said second part may be a temperature deviating from said predetermined value, or a supply of radiation that may deviate said temperature of said first part from said predetermined value when incident on said first part. Further, the second part may comprise a power dissipating element, which may be a part of a motor comprised in said apparatus. The motor may, for instance, be constructed and arranged to displace one of said mask table and said substrate table with respect to a frame of said apparatus, or to displace one of said mask holder and said substrate holder with respect to said mask table or said substrate table, respectively.
In a lithographic projection apparatus comprising a base frame and a metrology frame that is mounted substantially vibrationally isolated on said base frame, said metrology frame may be said first part. Said first part may also be a volume of gas in a projection apparatus comprising interferometric displacement measuring means having a beam of monochromatic coherent radiation traveling through the volume of gas of said apparatus.
According to a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a mask table provided with a mask holder constructed to hold a mask;
a substrate table provided with a substrate holder constructed to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate, said method comprising the steps of:
providing a mask bearing a mask pattern to said mask table;
providing a substrate which is at least partially covered by a layer of radiation-sensitive material to said substrate table;
arranging a heat shield such that a first part of said apparatus is shielded by said heat shield from a second part, said first part required to have a temperature of a predetermined value, and said second part having a characteristic that may influence said temperature of said first part;
controlling a temperature of said heat shield to said predetermined value; and
using the projection beam of radiation and said projection system to project an image of at least a portion of the mask pattern onto a target portion on the substrate.
In a manufacturing process using a lithographic projection, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.